In a vehicle equipped with electric power steering (xe2x80x9cEPSxe2x80x9d), the steering assist may be provided by an electric motor coupled to the steering column or shaft. In order to provide a stable and precise feel to the steering, it is desirable to provide input-dependent damping. An example of such a system is presented in U.S. Pat. No. 5,919,241 (xe2x80x9cthe ""241 patentxe2x80x9d), Vehicle Having Electric Power Steering With Active Damping, filed Dec. 13, 1996 and assigned to the assignee of the present application.
The ""241 patent shows input-dependent damping of a current-controlled EPS motor capable of actively damping the EPS response at higher vehicle speeds. At lower vehicle speeds, the inherent damping from mechanical and motor control sources was adequate to ensure stability and robustness since the motor controller included a hardware current loop that added damping by slope compensation to ensure stability of the current loop. Slope Compensation is a method well known in the art for stabilizing fixed frequency pulse width modulated closed loop current control, which involves the addition of a negative periodic triangular wave voltage to the reference voltage in the current control circuit. The period of the triangular wave voltage is equal to the pulse width modulation frequency. Slope compensation reduces the peak current allowed as the field-effect transistor (xe2x80x9cFETxe2x80x9d) duty cycles become longer, such as when the motor speed increases.
At higher vehicle speeds, the input-dependent damping algorithm added additional damping to achieve the desired free control response. Excessive amounts of input-dependent damping would have adversely affected the on-center feel and return performance of the EPS system since the damping acts as a brake to slow the system down when returning to center.
Although a current-controlled motor has an inherently controllable torque output, a voltage-controlled brushless motor has an inherently controllable angular velocity (xe2x80x9cspeedxe2x80x9d) output instead. A voltage-controlled brushless motor has certain advantages over a current-controlled motor that may be exploited to meet design criteria in an EPS system. Unfortunately, the torque output of a voltage-controlled brushless motor requires a more advanced controller than that required for a current-controlled motor in order to adequately damp the EPS system. Thus, it is desirable to provide at least one of input-dependent and assist-dependent damping of voltage-controlled brushless motors in EPS applications.
The above described and other features are exemplified by the following Figures and Description in which an electric power steering system is disclosed that includes a steering wheel, a voltage-controlled electric assist motor connected to the steering wheel or steering column, and an electronic controller electrically connected to the assist motor that receives a first signal representing a torque command to the assist motor and a second signal representing an angular velocity of the assist motor, and produces a voltage signal according to an assist-dependent damping function of the first and second signals, and uses the voltage signal to control the voltage-controlled electric assist motor in response to the assist-dependent damping function.